Electron discharge tube



Sept. 3, 1940. w. FLECHSIG ELECTRON DISCHARGE TUBE Filed April 21, 1938 Jrvuvnhw Patented Sept. 3, 1940 UNITED STATES ELECTRON DISCHARGE TUBE Werner Flechsig,

Berlin-Charlottenburg,

Ger-

many, assignor to the firm Fernseh Aktiengesellschaft, Zehlendorf, near Berlin, Germany Application April 21, 1938, Serial No 203,419 In Germany April 30, 1937 1 Claim.

This invention relates to electron discharge tubes, employing thermionic emission, which are operated in the region of the characteristics in wh ch a retarding field action upon the electron emission is exerted.

In operating thermionic electron discharge tubes in the retarding-field region, the difiiculty arises that the emitted current is highly dependent upon the temperature of the cathode, that is, upon variations in the filament heating power. This is particularly true of grid-controlled amplifier tubes, operated in the retarding-field region, in which the emission from the cathode is subjected to one or repeated processes of electron multiplication after passing through the control grid. Measurements show that a 10% variation of the heating power results in doubling, or halving, of the current in the retarding-field region.

It is the object of this invention to overcome this defect. It is known to vary the operating point of an amplifier tube along its characteristics by producing the grid bias by means of the voltage drop across a resistor caused by the emission from the cathode flowing through the resistor. These amplifier tubes are operated in the space-charge region, and greater emission may be caused by greater efficiency of the cathode or increase in plate voltage. An increase in emission will then produce a higher negative grid bias.

This method may also be readily applied in the case of control in the retarding-field region in order to maintain a constant average cathode emission for varying heating power. Because the grid current represents a considerable portion of the tota1 emission, if the tube possesses high mutual conductance in the retarding-field region, a sufiiciently large grid-leak resistance may fulfill the same purpose in this case. The tube characteristics in the retarding-field region follow exponential curves, and diificulties arise if large control amplitudes are applied. The curvature of the characteristics causes a rectifier effect, which largely displaces the automatically adjusted operating point. Amplitudes of only volt are to be considered large in this respect. Variations in the amplitude of the control Voltage would consequently lead to variations in the gain of the tube. Increases in amplitude having a duration shorter than the time constant'of the circuit components in the cathode lead would be amplified without displacement of the operating point. However, increases in amplitude of greater duration would lead to a displacement of the operating point, resulting in smaller gain.

This invention provides for substantially maintaining the position of the operating point in such cases also, without disturbing the described compensation for variations in cathode temperature. To this end, the minor portion of the cathode emission is subjected to grid control, while the major portion is not modulated and is collected by an auxiliary electrode which is connected with the cathode through a resistor. A negative grid bias is produced across this resistor which increases with increasing cathode current. This resistor is shunted by a condenser in the known manner. The fact that only a portion of the emission is utilized is no disadvantage, because small currents are an inherent factor of control in the retardingfield region, and these small currents can readily be amplified to any value desired by electron multiplication.

A greater amplitude of control Voltage causes the average emission flowing towards the control grid to increase. However, this now causes only a negligible increase in total emission current. Thus, the point of operation remains in substantially the same position. If, however, a variation in cathode temperature occurs, the entire emission current is varied so that the operating point adjusts itself to the correct position.

Thedrawing shows an embodiment of the invention. The unipotential cathode l is indirectly heated by a filament, not shown. The current emitted by the middle portion of the cathode alone is subjected to grid control. At this point the cathode is surrounded by control grid 2. The remaining (by, for instance, 80% or more) greater portion of the cathode is surrounded by two symmetrically arranged cylindrical electrodes 3 and 4, which are 'conductively connected to each other. connected to the cathode I through a parallel network 5 consisting of a resistor and a condenser, across which the grid bias is produced in the described manner. Between the auxiliary cylinders 3 and 4, and the grid 2, is connected a grid-leak resistor 6, which is sodimensioned that no substantial voltage drop is produced across it. The control voltage is applied to the grid through a condenser l. The elements of the electron discharge tube are housed in a vacuum receptacle, as shown at 8.

I claim:

In combination, an electron discharge tube comprising a cathode, a pair of cylindrical auxiliary electrodes disposed about the end portions of said cathode and exposing an intermediate portion of said cathode, a grid-like auxiliary electrode disposed about said intermediate portion only, means including a resistor external to said tube for connecting said cylindrical auxiliary electrodes to said cathode, and a separate circuit for applying signals to said grid-like auX- iliary electrode.-

WERNER FLECI-ISIG.

The cylinders 3 and 4 are" 

